The acoustic environment of automobiles and/or rooms (e.g., in a home, office, theatre, other building or enclosure, etc.) is difficult and may present challenges to reproduce a satisfying audio experience. For example, automobile audio system design and the acoustics of an automobile may contribute to the loss of bass. Loss of bass can be attributed to the inability of loudspeakers to reproduce lower frequencies along with a low frequency roll off attributed to factory speaker protection systems that limit bass output at higher listening levels. The physical location of the loudspeakers also becomes a factor in sound quality having an impact on the low frequency and high frequency response. Automobiles also work as sealed enclosures that produce mid bass resonance making the bass response unnatural sounding. Last the sensitivity of the ear at low frequencies is not the same at low levels as it is at higher levels as explained by Harvey Fletcher et al., Loudness, Its Definition, Measurement and Calculation, in Journal of the Acoustic Society of America, Vol. 5, October 1933. It is widely known that the low frequency response in an automobile given the many factors of speaker placement, listening position and automobile resonance, make it quite difficult to make the automobile audio systems sound equivalent to that of a high end stereo system.
It is generally accepted that the interior of an automobile is far from being an ideal listening space. One of its most obvious flaws is a dominant resonance at 250-300 Hz that makes the bass lose tonality and unnatural sounding. This resonance reinforces the mid bass produced by a typical sound system in an automobile and while this resonance can be used to an advantage in competitive high SPL autosound competitions is not helpful to the objective of reproducing good sound in an automobile
Several approaches have been taken to compensate for low frequency roll off in an automobile environment. Graphic equalizers are typically used to make the necessary compensation that is usually done after measurements have been made with a ⅓-octave real time analyzer. This requires the end user to have some skill and audio knowledge to use the analyzer and then apply that to the equalizer.
The concept of an accelerated slope tone control for bass compensation was introduced by Dennis A. Bohn in Accelerated Slope Tone Control Equalizers, J. Audio Eng. Soc., Vol. 40, No. 12, December 1992, and U.S. Pat. No. 5,046,105 issued Sep. 3, 1991 describes a system that improved the function of typical bass and treble tone controls. This concept demonstrated the use of 2nd and 3rd order filters with an added zero offset for each additional pole to eliminate phase cancellations in the mid bass. The advantage of the accelerated slope tone control concept is to reduce mid band interaction inherent in bass and treble tone controls.
Additionally, numerous methods exist that improve the bass response of smaller loudspeakers with limited bandwidth. The most significant improvement in sound reproduction in a vehicle is to typically add bass loudspeakers and power amplification.
Further, audio quality in an acoustic environment can be affected by the geometry of a listening space. For example, acoustic environments may have null regions caused by standing waves. Additionally, some devices used in room environments may use bass roll off to protect small speakers included in or used by the device to produce audio. Thus, what is needed, is a method to optimize the use of the aforementioned improvements in acoustic environments, such as, for example, a vehicle environment and/or a room environment.